This invention relates to an index signal amplifier for use in a beam index color television receiver and, more particularly, to such an index signal amplifier which can exhibit standardized characteristics so as to be used in various beam index color television receivers.
A beam index color television receiver is provided with a cathode ray tube (CRT) having a single electron beam which scans the color phosphor elements provided on the display screen of the CRT, the scanning electron beam being modulated with the appropriate color information as the beam scans the respective color elements. This differs from a conventional color CRT in which three individual electron beams simultaneously scan the color phosphor stripes, with each beam being modulated with its associated color information.
In the beam index color television receiver, an index signal is generated in synchronism with the scanning of the electron beam, this index signal being used to control a color switching circuit whereby the proper color information is "switched" to modulate the single beam at the proper time. That is, when the single beam reaches the red element, red information is switched to modulate the beam intensity; and then when the beam reaches the green element, the green information is switched to modulate the beam intensity; and then when the beam reaches the blue element, the blue information is switched to modulate the beam intensity. In order to generate the index signal for controlling the color switching operation, the CRT also is provided with index stripes, such as special phosphor stripes, disposed at or in the vicinity of the display screen, whereby the index stripes are scanned as the beam scans the color phosphor elements. These index stripes are uniformly spaced, and each is excited by the scanning beam so as to emit light when the beam is incident thereon. A photodetector is disposed to receive the light emitted by each of the scanned index stripes, this photodetector generally being provided on the envelope of the CRT. Hence, as the beam scans a raster, the photodetector generates an index signal having an index frequency which is a function of the scanning speed of the beam and the pitch of the index stripe. This index signal derived from the photodetector is used to control the color switching operation whereby the proper color information is switched to modulate the beam intensity at the proper times during each scanning trace of the beam.
In general, the amplitude of the index signal produced by the photodetector is too low to effect the proper control over the color switching operation. Hence, it is necessary to amplify that index signal. This amplification may be performed by a band-pass amplifier whose center frequency is equal to the expected index frequency. However, for proper amplification of the generated index signal, it is necessary that the gain of this amplifier be relatively high, such as on the order of about 60 dB. At the relatively high frequency of the index signal, the high gain of this amplifier results in an undesired feedback effect, whereby the amplified index signal is injected back into the input of this amplifier, thereby causing regenerative feedback. This, in turn, results in undesired oscillation of the amplifier. Such oscillation substantially interferes with the proper index signal, thereby introducing errors into the color switching arrangement for the CRT.
Another disadvantage of providing a simple amplifier for amplifying the index signal generated by the photodetector is that the frequency of the index signal may vary from one tube to another. For example, if the size of the CRT, that is, the display screen of the CRT, is increased, the scanning speed of the electron beam likewise is increased; and if the pitch of the index stripes remains the same, the frequency of the index signal is increased. As a result thereof, the index frequency no longer coincides with the center frequency of the band-pass amplifier. Hence, a separate band-pass amplifier must be provided for this larger cathode ray tube. It is, therefore, appreciated that different band-pass amplifiers must be provided for the different sizes of CRT's which can be used in the beam index color television receiver. Furthermore, even with CRT's of the same size, small differences in the physical parameters from one tube to another may result in correspondingly different index frequencies, thereby requiring band-pass amplifiers tuned to different center frequencies. It should be recognized that these disadvantages prevent the desired standardization of the band-pass amplifier for use in all beam index CRT's. Consequently, since different amplifiers must be designed for the different tubes, the cost of manufacturing and assembling such beam index color television receivers is relatively high.